The septins are a novel family of proteins recently discovered in budding and fission yeast, insects (Drosophila), and mammals (both mouse and human). The available immunolocalization and genetic data make clear that septins are involved in cytokinesis both in the yeasts and in Drosophila (despite the apparent differences in modes of cytokinesis) and suggest strongly that they also have other roles in the organization of the cell surface in nondividing cells. The long-term goal of this project is to exploit the experimental advantages of Drosophila (particularly its susceptibility to genetic analysis) to elucidate the roles of these proteins in an animal system. Such studies should provide information about the mechanisms of cytokinesis, the interactions of cytoskeletal elements with each other and with the plasma membrane, and other aspects of the organization of the cell cortex. Specific Aims for the initial project period include; (1) Clarification of the role of the septins in ordinary cytokinesis by more detailed cytological analysis of two known septins (Sep1 and Pnut) and more detailed analysis of the cytokinesis defect in mutants lacking Pnut; (2) investigation of the apparent role of Sep1 and Pnut in the cellularization of the embryo, using similar approaches; (3) attempts to generate hypotheses about the roles of septins in nondividing cells by more detailed examination of the localization of Sep1 and Pnut in such cells; (4) initial biochemical studies of Sep1 and Pnut; (5) attempts to generate mutants defective in Sep1 and additional mutations affecting Pnut, to explore the functional overlap and interactions of these proteins, and to identify interacting proteins genetically; and (6) initial studies of the functions of a third Drosophila septins (Sep2). In future project periods, the analysis of Sep2 function will be extended, the identification nd analysis of additional Drosophila septins will be undertaken, attempts to identify and analyze the functions of interacting proteins will be continued, the roles of the septins in nondividing cells will be explored in more detail, and detailed structure-function studies of the septins will be undertaken. Cytokinesis is an essential part of the normal cell cycle and as such is crucial for normal cell proliferation and tissue development; thus, defects in cytokinesis may be important in diseases, such as cancer, that involve abnormal cell proliferation and development. Moreover, the proper organization of the cell cortex is essential for the normal organization of the cytoplasm, cell-cell adhesion, and intercellular communication, all processes that are essential for normal cell function and defective in various disease states. Thus, studies of the novel septin protein family in an experimentally tractable model system should contribute significantly to understanding both of normal cell and developmental biology and of important diseases.